Partition coefficient difference of benzocarbazoleisomers between oil, water and mineral phase makesthem a useful indicator to quantify petroleum migrationdistance. Because of their nitrogen-heteroatom andannelated aromatic cycles they are generally regarded asbeing more resistant and the effects of biodegradation ontheir concentrations and distributions have not previouslybeen investigated. Reservoir extracts from three wells locatedin the Leng 43 block of the Liaohe Basin were analyzedto investigate their occurrence and the effect of biodegradation.Both hydrocarbon biomarkers and benzocarbazoleisomers show systematical changes with the increase extentof biodegradation in study columns. Carbazole compoundsmay be biodegraded in a similar way to that observed inaliphatic and aromatic hydrocarbons. The distance from oilwater contact is a primary control factor for biodegradation.The concentrations of benzocarbazole isomers show a slightincrease in the upper part of the columns then a sharp decreasetowards oil water contact (OWC). Among three isomersbenzo[a]carbazole seems more susceptible to biodegradationthan other two isomers and benzo[b]carbazole hashigher ability to resist bacterial attack. Benzo[b]carbazole/benzo- [a]carbazole ratios can sensitively indicate thedegree of biodegradation and the benzocarbazole index (BCratio) cannot be directly used as a migration indicator inbiodegraded oils.